Recently, various antibody drugs for treating cancers by targeting antigen proteins on cancer cells have become up in the world. The antibody drugs show certain beneficial effects as cancer-specific therapeutic agents and have received attention. However, most of the target antigen proteins are expressed also on normal cells, and administration of such an antibody impairs not only cancer cells but also normal cells expressing the antigen, resulting in a problem of side effects therefrom. Accordingly, if a cancer antigen being specifically expressed on cancer cell surface is identified and an antibody targeting the antigen can be used as a pharmaceutical agent, treatment with an antibody drug with less side effects can be expected.
It is known to those skilled in the art as general technical knowledge that although the mortality of liver cancer is being gradually decreased, among various cancers, the rate of deaths from liver cancer is still high, the fourth place in deaths from cancer classified according to the sites thereof in Japan, to be difficult to be treated. Accordingly, it is desired to develop an effective therapeutic agent for liver cancer.
Cytoplasmic- and proliferation-associated protein 1 (CAPRIN-1) has been known as an intracellular protein that is expressed in activation of normal cells in the resting phase or in occurrence of cell division and is involved in control of transport and translation of mRNA through formation of intracellular stress granules with RNA in cells. It was found that CAPRIN-1 is specifically expressed on the surface of cancer cells such as breast cancer cells, and CAPRIN-1 has been studied as a target of antibody drugs for cancer therapy (Patent Literature 1). However, in Patent Literature 1, expression of CAPRIN-1 on liver cancer cells is not recognized, and it is not described or suggested that CAPRIN-1 can be an antigen protein of liver cancer.